A swivel work machine such as a backhoe employs, as a traveling device, a crawler traveling device having crawlers comprised of rotatable endless belts on the right and left. And, this crawler traveling device includes a track frame as a device body. The track frame usually includes a center frame having a swivel bearing which rotatably supports a swivel frame mounting thereon an engine, an engine hood, etc. and a pair of right and left side frames connected to the right end left ends of the center frame and extending in the fore/aft direction. Each side frame includes, at its front and rear ends, an idler and a drive wheel about which a crawler is wound.
As this type of track frame, there is known one wherein the center frame is formed by welding and joining upper and lower plate members formed of metal plates to each other through a plurality of wall plate members. Further, on the lateral side of this center frame, these plate members form a plurality of leg portions and the side frame is joined to the leading ends of these leg portions.
If the center frame is constructed of metal plates as described above, it is needed to assemble and weld the plate members having complicated shape and/or bend with each other under mutual linear contact condition thereof. As a result, there tends to occur stress concentration or irregularity in the welding quality. On the other hand, if the thickness of each plate member is increased to ensure the strength, this can invite disadvantageous weight increase.
As a swivel work machine having a track frame capable of avoiding the above problem, Patent Document 1, for instance, discloses a swivel work machine in which the track frame is formed by joining the center frame and the side frame through connecting legs provided in pairs on the front and rear sides and on the right and left sides (4 legs in total).    Patent Document 1: Japanese Patent Application “Kokai” No. 2005-1652